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2027856847
This reverts commit 45f5efb9ba.
The load_and_repair_paxos_state function was introduced in
scylladb/scylladb#24478, but it has never been tested or proven useful.
One set of problems stems from its use of local data structures
from a remote shard. In particular, system_keyspace and schema_ptr
cannot be directly accessed from another shard — doing so is a bug.
More importantly, load_paxos_state on different shards can't ever
return different values. The actual shard from which data is read is
determined by sharder.shard_for_reads, and storage_proxy will jump
back to the appropriate shard if the current one doesn't match. This
means load_and_repair_paxos_state can't observe paxos state from
write-but-not-read shard, and therefore will never be able to
repair anything.
We believe this explicit Paxos state read-repair is not needed at all.
Any paxos state read which drives some paxos round forward is already
accompanied by a paxos state write. Suppose we wrote the state to the
old shard but not to the new shard (because of some error) while
streaming is already finished. The RPC call (prepare or accept) will
return error to the coordinator, such replica response won't affect
the current round. This write won't affect any subsequent paxos rounds
either, unless in those rounds the write actually succeeds on both
shards, effectively 'auto-repairing' paxos state.
Same if we managed to write to the new shard but not to the old shard.
Any subsequent reads will observe either the old state or the new
state (if the tablet already switched reads to the new shard). In any
case, we'll have to write the state to all relevant shards
from sharder.shard_for_writes (one or two) before sending rpc
response, making this state visible for all subsequent reads.
Thus, the monotonicity property ("once observed, the state must always
be observed") appears to hold without requiring explicit read-repair
and load_and_repair_paxos_state is not needed.
Closes scylladb/scylladb#24926